D lang compilers have an option -release (or similar) which turns off
asserts, contracts, and other runtime type checking. This patch wires
that up to the b_ndebug flag.
Fixes#7082
Also add MSVC support while doing so. Doesn't test 32-bit Windows yet
because I can't figure out how to do manual setup for function calls on
Win32 which also does symbol mangling. But this is fine for now.